zcash_client_backend: Generalize & extend wallet metadata query API #5980

Job	Run time
Build target wasm32-wasi	52s
Check ZFuture build on Linux	27s
Check ZFuture build on macOS	35s
Bitrot check	1m 32s
Check ZFuture build on Windows	3m 8s
Latest build on ubuntu-latest	1m 33s
Latest build on windows-latest	1m 25s
Clippy (MSRV)	1m 5s
Latest build on macOS-latest	1m 25s
Clippy (beta)	14s
Code coverage	4m 1s
Intra-doc links	2m 6s
Rustfmt	15s
protobuf consistency	1m 2s
Test on Linux	1m 37s
UUID validity	3s
Test Orchard on Linux	45s
Test NU6 on Linux	1m 43s
Test on macOS	1m 10s
Test Orchard on macOS	8s
Test on Windows	1m 40s
Test Orchard on Windows	1m 25s
Test NU6 on Windows	1m 33s
	29m 44s

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